Proton-induced permeability and fusion of large unilamellar vesicles by covalently conjugated poly(2-ethylacrylic acid)

Proton sensitive large unilamellar vesicles (LUV) were constructed by immob ilization of the pH sensitive synthetic polymer poly(2-ethylacrylic acid) o nto the outer monolayer. Thiolated poly(2-ethylacrylic acid) (PEAA-SH) was covalently conjugated to the surface of LUVs composed of egg phosphatidylch oline (EPC) and cholesterol (Chol) through the thiol-reactive maleimide lip id MPB-DSPE (N-(4-(p-maleimidophenyl)butyryl)-1,2-distearoyl-sn-glycero-3-p hosphoethanolamine). The resulting PEAA-LUVs were shown to be stable at neu tral pH (pH 7.0 to 8.0). Under acidic conditions, however, protonation of P EAA resulted in interaction with both the membrane it was linked to and the membrane of target vesicles, causing membrane destabilization and release of vesicle contents. Moreover, conjugated PEAA is shown to mediate fusion w ith target membranes in a pH dependent manner. PEAA-mediated permeabilizati on and vesicle-vesicle fusion occurred only when the polymer was covalently linked to the LUV surface. Proton dependent fusion of PEAA-LUVs was also o bserved with erythrocyte ghosts. This pH-dependent release of vesicle conte nts and fusion of PEAA-LUVs occurred below pH 6.8, which is well within the pH range expected to be encountered inside the endosomes in the endocytic pathway, indicating the potential of PEAA-LUVs as a drug carrier system for intracellular drug delivery.